This invention is in the field of polymeric composites; more particularly, the invention relates to a stampable thermoplastic composite containing a shielding layer which can be stamped and provides effective shielding against electromagnetic and radio frequency radiation.
Electronic equipment can emit a variety of electromagnetic waves and radio frequency waves causing interference commonly called electromagnetic interference (EMI) and radio frequency interference (RFI). Efforts are continually under way to shield the various sources to prevent the EMI/RFI from affecting nearby electronic equipment. A common way to shield electronic equipment is to encase it in a material that shields electromagnetic and radio frequency waves.
One alternative is to locate electronic equipment in rooms that are shielded against the transmission of electromagnetic and radio frequency waves. This is sometimes difficult with smaller electronic apparatus and it is more desirable to localize the shielding around the particular apparatus. Various apparatus have been shielded by encasing them in metal containers. However metal containers tend to have difficulty being shaped, conduct electricity themselves, and can present a weight problem. As a result, new materials have been developed for shielding a variety of shaped electronic equipment including various electronic apparatus and cables. One product described is amorphous metal or metal glass. Various forms of this material can be shaped by rolling and stamping for use as EMI/RFI shielding material. Such material is discussed in Electronics, p. 138, Mar. 3, 1977 and in U.S. Pat. No. 4,189,618.
Conductive plastics are also used to make a variety of shaped EMI/RFI shields. Reference is made to Simon, R.M. EMI Shielding Through Conductive Plastics, Polym.-Plast. Technol. Eng., 17(1), 1-10 (1981). This article discusses EMI shielding using conductive plastics. Included in this article is a discussion of the fact that conductive fillers can be used such as powdered metal and carbon. This article additionally notes that graphite fibers provide a shielding advantage as well as a high strength reinforcement. This article also recognizes the use of metalized glasses in chopped form, roving, or mat. Finally, the article notes a fibrous conductive filler which is a pure metal or metal alloy flake or fiber. It is disclosed in this article that mat molding compounds can contain 5% of a flake product to give shielding in a 30 (decibel) dB range. It is also disclosed that flake and fiber based conductive polymers have been made using polypropylene, polycarbonate, nylon 66, nylon 6, nylon 12 and phenolic resins.
Modern Plastics, p. 58, June 1982 discloses conductive sheet molding compounds (SMC) shields against EMI/RFI. A particular disclosure is a sheet molding compound reinforced with 22% glass fiber, carbon fiber mat, conductive fillers and conductive carbon pigment. The use is for a housing for a computer printer.